Liquid optically clear adhesives are used in many electronic devices including: in touch sensitive panels, display devices such as computer monitors, mobile phone screens, tablet screens and television screens. The past technology included die-cut double sided film adhesives or optically clear adhesive tapes which were not entirely satisfactory. The advantages of liquid optically clear adhesive (LOCA) include re-workability, adhesion to uneven surfaces, better optical properties, and durability. The use of a LOCA also allows for thinner designs compared to tapes. One of the other past issues with uses of double-sided tapes is the creation of a bezel around the liquid crystal module (LCM), for example a mobile phone screen, caused by the tape. As devices become more complex the desire to enhance the viewable area of a given screen increases such that any change that can reduce the bezel results in a bigger viewing area without increasing the overall size of the screen. In addition, while these tapes are “optically clear” they are not necessarily ideally matched to the refractive index of the other components in the LCM which can reduce brightness, clarity, and the contrast ratio. In addition, use of a tape introduces an air gap between the components in the LCM which can further interfere with a clear display by introducing increased reflected light. The air gap can also allow for condensation to develop between the bonded layers.
Many of the current LOCA utilize silicone-based adhesives as part of their formulations. Often times these silicones are prepared by the known sol/gel processes to form a variety of MTQ type silicone resins. Throughout the present specification and claims the standard short hand definitions of silicone nomenclature will be used. Under these definitions: M type units are monofunctional R3SiO; D type units are difunctional R2SiO2; T type units are trifunctional RSiO3; and Q type units are tetrafunctional SiO4. These sol/gel prepared silicone resins are often subjected to a second process wherein they are fluorinated to provide enhancements to their properties, including lowering their refractive index. This second process proceeds through reaction of fluoro containing reagents with the residual silanol groups in the sol/gel prepared resin and thus the process is multi-step and the fluorination reactions can be difficult to control. The amount of incorporation of fluorine is hard to control since it depends on the amount of residual silanol groups in the silicone resin.
Another process that has been used to lower the refractive index of adhesives is to include addition of fluorinated acrylate monomers with the adhesive composition to lower their refractive index; however these suffer from a number of issues. The fluorinated acrylate monomers tend to be very volatile and thus one is required to introduce oligomeric or polymeric species into the adhesive formulations to reduce the volatility of that adhesive formulation. The introduction of the oligomeric or polymeric species causes an undesirable increase in the refractive index of the adhesive defeating the purpose of including the fluorinated acrylate monomers. In addition, the use of highly fluorinated acrylates tends to reduce the adhesion value of the adhesive due to the fluorine units in the fluorinated acrylate monomer. Thus, there exists a need to find alternative ways or sources of materials that can be used to lower the refractive index of adhesives, particularly LOCA type adhesives, without reducing the adhesive properties and the other desirable properties of the adhesive.
It is desirable to provide a LOCA that has a very low refractive index, a low viscosity, and high adhesion allowing for use of very thin layers of the adhesive and to provide a thin overall design. It is furthermore desirable to provide a method for formation of highly fluorinated silicone resins in a simplified single step process for use in a LOCA. It is also preferred to produce a highly fluorinated silicone resin that can be radically crosslinked to improve strength and durability. Preferably the LOCA can bond to a variety of substrates and does not exhibit shrinkage, haze or color on curing.